1. Field of the Invention
This invention relates to a sparking voltage detecting device for spark-ignition type internal combustion engines, which detects sparking voltage generated in the engine.
2. Prior Art
Conventionally, sparking voltage detecting devices are known, which detect sparking voltage occurring on the secondary side of an ignition system of an internal combustion engine. One of such devices is disclosed in Japanese Provisional Patent Publication (Kokai) No. 52-118135, which comprises a sensor formed of a strip of a conductive material wound on a high-tension cord connected to a spark plug. The sensor forms a capacitor having very small capacitance with a coating insulating material of the high-tension cord acting as a dielectric, the conductive material strip being serially connected to a measuring capacitor which is grounded at one end thereof. A signal voltage is generated across the measuring capacitor according to the principle of electrostatic potential division, which voltage is detected as the sparking voltage.
However, although the known sparking voltage detecting device is capable of ascertaining whether or not sparking voltage is occurring, it is unable to accurately detect waveforms of sparking voltage, because the device has a simple structure formed of a conductive material strip wound on the high-tension cord, and hence is liable to have changes in the electrostatic capacity due to vibrations, changes in the temperature (humidity), etc. As a result, the measured voltage largely varies, badly affecting the detecting accuracy and the effective life of the device. Moreover, the sensor is located close to a voltage source (high-tension cord) generating several tens thousands volts, which requires the use of an expensive protective circuit for effecting a failsafe action in the event of a high voltage leak, and hence is complicated in structure.